Acoustic wave devices are used as filter elements and oscillators of wireless devices for example. There has been known a surface acoustic wave (SAW: Surface Acoustic Device) device as a device using acoustic waves. The SAW device is used to a variety of circuits processing radio signals in a frequency of 45 MHz to 2 GHz such as a transmission bandpass filter, a reception bandpass filter, a filter for a local oscillator, an antenna duplexer, an IF (Intermediate frequency) filter, and an FM (Frequency Modulation) modulator.
The SAW device has an IDT (Interdigital Transducer) including two opposing comb-shaped electrodes. The comb-shaped electrode has a bus bar and electrode fingers extending from the bus bar to a same direction and exciting acoustic waves. The electrode fingers of the opposing comb-shaped electrodes are arranged in an alternate order. The acoustic wave is propagated to a direction intersecting with an extension direction of the electrode fingers. However, generated is a lateral-mode spurious caused by the acoustic wave propagated to a direction of the electrode fingers. There has been examined a method that provides a dummy electrode of one comb-shaped electrode to a tip of an electrode finger of another comb-shaped electrode so as to form a gap, and scatters an acoustic wave in a lateral mode by making a length of the dummy electrode different from a length of a dummy portion of a cross electrode arranged adjacent to the dummy electrode in order to suppress the spurious as disclosed in International Publication Pamphlet No. WO 2006/078001. There has been known a method that forms a dielectric film on an electrode finger but does not form a dielectric film on a bus bar to suppress the spurious as disclosed in International Publication Pamphlet No. WO 2008/078573.
The acoustic wave to be excited is distributed to a dielectric film in an acoustic wave device in which the dielectric film is formed on a substrate. Thus, the method of Patent Document 1 scatters a part of the acoustic wave in the lateral mode in a boundary face between the substrate and the dielectric film. However, it is difficult to scatter the acoustic wave within the dielectric film. Therefore, the suppression of the spurious is not sufficient. In addition, when the method of the Patent Document 2 is used, a surface of the bus bar and the like is exposed from the dielectric film. This may decrease a reliability.